In recent years, various types of flat panel display devices, which have light weight and small size compared with cathode ray tube (CRT) displays, have been developed. Among the various types of flat panel display devices, by using a self-light-emitting organic light-emitting diode (OELD) to display images, an active matrix organic light-emitting display device with a thin-film transistor (TFT) backplane usually has the characteristics of short response time, low power consumption for driving, and better brightness and color purity. Therefore, the organic light-emitting display device has become a focus of the next-generation display devices.
FIG. 1 schematically shows a circuit diagram of a traditional active matrix organic light-emitting display device 100, wherein the active matrix organic light-emitting display device 100 comprises a data driver and a scanning driver (not shown in FIG. 1). The data driver is configured to control a plurality of data lines DA1 . . . DAm in transversal arrangement, and the scanning driver is configured to control a plurality of scanning lines SC1 . . . SCn in longitudinal arrangement. A plurality of pixel circuits 110 are formed in intersection areas between the plurality of data lines DA1 . . . DAm and the plurality of scanning lines SC1 . . . SCn.
With reference to FIG. 1, the pixel circuit 110 comprises an organic light-emitting diode (OLED)1, a storage capacitor C11, a switching transistor T11, a driving transistor T12, a first power source ELVDD1, and a second power source ELVSS1, wherein both the transistors T11 and T12 are P-channel metal-oxide semiconductor transistors (PMOS). A grid of the switching transistor of the switching transistor T11 is coupled to one scanning line SC1, a source of the switching transistor T11 is coupled to one data line DA1, and a drain of the switching transistor T11 is coupled to a grid of the second transistor T12. A source of the driving transistor T12 is coupled to the high-voltage power source ELVDD1, and a drain of the driving transistor T12 is coupled to an anode of the OLED1. A cathode of the OLED1 is coupled to the low-voltage power source ELVSS1. A first terminal of the storage capacitor C11 is coupled to the first power source ELVDD1, and a second terminal of the storage capacitor C11 is coupled to the grid of the second transistor.
The scanning driver applies scanning signals to the scanning lines SC1 to SCn in sequence, and the data driver applies corresponding data signals via the data lines DA1 to DAm according to image data to be displayed. Thus, the pixel circuits 110 located in the intersection areas supply a driving current flowing through the organic light-emitting diode according to the signals of the scanning lines and data lines coupled to the pixel circuits.
Using the pixel circuit 110 shown in FIG. 1 as an example, when the scanning drive applies the scanning signals to the scanning line SC1, the switching transistor T11 is conducted, and at this point, a voltage of the data signals on the data line DA1 is stored in the storage capacitor C11 through the switching transistor T11. The driving transistor T12 supplies a driving current IOLED1 according to the voltage stored in the storage capacitor C11 to drive the organic light-emitting diode OLED1 to emit the light of the corresponding brightness. A formula for the driving current is shown as below:IOLED1= 1/12μ12×Cox12×W12/L12(VGS12−VTH12)2  (Formula 1),wherein μ12 is a carrier mobility of the driving transistor T12, Cox12 is a capacitance of a control end oxidation layer per unit area of the driving transistor T12, W12 is a channel width of the driving transistor T12, L12 is a channel length of the driving transistor T12, VGS12 is a voltage difference between the grid and the source of the driving transistor T12, and VTH12 is a threshold voltage of the driving transistor T12. That is, the driving current flowing through the organic light-emitting diode OLED1 can be controlled according to the magnitude of a data voltage from the data line DA1 to display a predefined grayscale.
A large active matrix organic light-emitting display device comprises a number of pixel circuits, and each of which need to comprise a driving transistor. The electric difference among different driving transistors results in different threshold voltages on the driving transistors. Therefore, according to the formula 1, it can be known that when the data voltages supplied to the pixel circuits 110 are the same, the driving currents supplied to the organic light-emitting diodes may vary with different threshold voltages of the driving transistors. This will result in the problems of poor quality uniformity and poor consistency of an image displayed by a plurality of pixel circuits.